This invention relates generally to broad beam electron guns and, in particular, to an improved hot cathode for broad beam electron guns. In the past, broad beam electron guns have called for the use of a shaping grid for regulating the dispersal of electrons being emitted from hot cathodes. Reference is made to Farrell, et al. U.S. Pat. No. 3,863,163, wherein a plurality of elongated filament cathodes are spaced apart and parallel to each other in a common plane and shaping grids having the shape of half-cylinders are disposed about the cathode filaments. An electrical potential is applied between the filament and the shaping grid so that electrons emitted from the cathode filaments are attracted toward the shaping grid in a corresponding pattern. A second potential is applied between the shaping grid and the anode so that after the electrons are attracted toward the shaping grid, they are then accelerated toward the anode with the desired energy level. The dispersal pattern created by the shaping grid provides the desired large cross-sectional area electron beam.
In other devices where multiple cathodes have been used, but without a shaping grid, the electron beams obtained did not have a uniform cross-sectional intensity.
While the use of a shaping grid provides the desired uniform beam intensity and large cross-sectional area, the addition of a shaping grid to an electron gun apparatus gives rise to additional complexity, added parts requirements, configuration modifications, as well as higher costs.
The elongated filaments used in Farrel present additional, practical problems. In Farrell, long filament lengths were required to obtain the broad electron beam cross-sections. For example, a 10-centimeter beamwidth requires a minimum of 10-centimeter filament lengths. Shorter filaments would be unsatisfactory since there is insufficient electron emission in the area of the ends of each filament in a direction parallel to the axis of the filament. Long filament lengths are difficult to physically support within the electron gun housing. As evidenced by the material in Farrell, additional parts are required to compensate for thermal expansion of the elongated filament, otherwise, sagging and resulting misalignment of the filament occur which, in turn, affect the uniformity of the electron beam. Additionally, long filament lengths require more power to operate than do filaments of shorter length.
Additionally, in the past, multiple cathode configurations used in broad beam electron guns often required precise geometries. This requirement arose because the broad electron beam was generated by summing a number of individual, narrow electron beams which were independent of one another. The parameters for each beam were such that the self-fields between the beams had insignificant effect upon the characteristics of the resulting broad beam. Therefore, any non-uniformity in any particular narrow beam caused a corresponding non-uniformity in the broad electron beam.
Under the above conditions, the spacings between cathode filaments, as well as the geometries of each cathode filament must be maintained with precision; otherwise, imbalances in beam cross-sectional distribution, as well as beam intensity, will occur. If, for example, a particular cathode had dimensions different from adjacent cathodes, the electron distribution pattern in the area corresponding to the differently dimensioned cathode would be different from the distributions in areas where the cathodes were of uniform dimension.
Thus, in multiple cathode broad beam electron guns of the past, precise geometries in the cathode configurations were necessary for satisfactory performance.